1. Field
The present disclosure relates to illumination devices. More particularly, the disclosure relates to powering solid state light emitting devices with an alternating current (AC) voltage source.
2. Background
Solid state light emitting devices, such as light emitting diodes (LEDs), are attractive candidates for replacing conventional light sources such as incandescent, halogen and fluorescent lamps. LEDs have substantially higher light conversion efficiencies than incandescent and halogen lamps and longer lifetimes than all three of these types of conventional light sources. In addition, some types of LEDs now have higher conversion efficiencies than fluorescent light sources and still higher conversion efficiencies have been demonstrated in the laboratory. Finally, LEDs contain no mercury or other potentially dangerous materials, therefore, providing various safety and environmental benefits.
More recently, solid state devices have been used to replace high-intensity discharge (HID) lamps to provide high levels of light over large areas when energy efficiency and/or light intensity are required. These areas include roadways, parking lots, pathways, large public areas, and other outdoor applications. To increase the intensity of light in these applications, often more than one solid state light emitting device is arranged in a package. An example of a solid state light emitting device is a light emitting semiconductor chip comprising a p-n junction. An example of a package is a collection of light emitting devices arranged on a substrate and encapsulated in a phosphor to produce broad spectrum white light. This package is sometimes referred to as an “LED array.” A heat sink is often attached to the LED array to dissipate heat generated by the light emitting devices.
LEDs are being used to replace high intensity discharge (HID) lamps in legacy street lights. The conversion to LED-based street lights generally involves replacing the existing head portion attached to the top of a pole with a new LED-based head. Because LEDs are diodes, when driven by AC power, light is emitted only during the forward bias phase of the voltage. In addition, because the duty cycle operation of the LED is equal or less than 50%, flicker can occur, and can be an unpleasant effect, and may pose an adverse safety situation for street traffic. Consequently, it is desirable to have an LED street light illumination system that overcomes duty cycle and flicker.